Reinforced concrete generally comprises a concrete mix, including the usual hydraulic cement and aggregate, in which reinforcing members are embedded and hence become invisible to the naked eye so that subsequent work with the reinforced concrete structure which requires penetration may damage the reinforcing members or may be interfered with by the reinforcing members.
The reinforcing members may be in the form of individual reinforcing bars (rebar), which can be bent or straight, rebar arrays interconnected by spacers or transverse members in one or more layers, wired together or relatively free from one another, reinforcing mats, baskets or the like. These members may be stressed, i.e. held under tension in the case of prestressed concrete structures, or simply laid into the concrete.
It is important, e.g. for penetrating the concrete structure without interference or damage to the reinforcement, to be able to ascertain from a surface of the structure the location, orientation and pattern of the reinforcing members, these parameters being collectively referred to herein as the disposition of the reinforcement.
It has already been proposed to determine the disposition of reinforcing members in a finished concrete structure by the use of so-called metal detectors which can include an electromagnet generating an electromagnetic field and means responsive to the distortion of the field caused by the presence of the ferromagnetic or predominantly iron reinforcing elements. However, experience has shown that in relatively thick concrete structures, in which the reinforcements are disposed 10 cm or more from the surface, the metal detector technique does not work. Furthermore, even at a distance of less than 10 cm, accurate determinations cannot be made if the reinforcements are relatively close together because the magnetic field distortion becomes diffuse.
For embedded depths of 10 cm or more, e.g. up to 60 cm, it is possible to use hard gamma radiation which is disadvantageous since both the source of the radiation and the detector are then comparatively expensive and may involve dangers to operating personnel.
Experiments with ultrasonics for the same purpose have demonstrated that these techniques are not effective because the sharp edges and facets of the aggregate reflect the ultrasonic waves in diverse ways to interfere with the ability to accurately detect the disposition of the reinforcing elements.